This invention relates to textile manufacturing. More specifically, this invention relates to stitching machines that are computer numerically controlled.
Large aircraft structures such as wing covers are now being fabricated from textile composites. The textile composites are attractive because of their potential for lowering the cost of fabricating the large aircraft structures. Cutting pieces of fabric and stitching the fabric pieces together have the potential of being less expensive then cutting sheets of aluminum, drilling holes in the aluminum sheets, removing excess metal and assembling metal fasteners.
The wing cover can be made from a carbon-fiber textile composite. Sheets of knitted carbon-fiber fabric are cut out into pieces having specified sizes and shapes. Fabric pieces having the size and shape of a wing are laid out first. Several of these pieces are stacked to form the wing cover. Additional pieces are stacked to provide added strength in high stress areas. After the fabric pieces are arranged in their proper positions, the pieces are stitched together to form a wing preform. Secondary details such as spar caps, stringers and intercostals are then stitched onto the wing preform. Such a wing preform might have a thickness varying between 0.05 inches and 1.5 inches. The wing preform is quite large, and its surface is very complex, usually a compound contoured three-dimensional surface.
The wing preform is transferred to an outer mold line tool that has the shape of an aircraft wing. Prior to the transfer, a surface of the outer mold line tool is covered with a congealed epoxy-resin. The tool and the stitched preform are placed in an autoclave. Under high pressure and temperature, the resin is infused into the stitched preform and cured. Resulting is a cured wing cover that is ready for assembly into a final wing structure.
The stitches are made by a computer numerically controlled (CNC) stitching machine. The stitching machine is programmed in a language that is native to CNC machines. On a wing preform, the stitching machine might make eight to ten stitches per inch in rows that might be spaced 0.1 inches to 0.5 inches apart over a surface that might be longer than forty feet and wider than eight feet. The total number of stitching points might exceed 1.5 million.
Generating code for the CNC stitching machine is slow, inefficient, and labor-intensive. At least one CNC instruction is generated for each stitching point on the wing preform. Even though CAD/CAM systems can automatically generate toolpaths, which constitute much of the code, the CAD/CAM systems do not generate all of the code. For example, the CAD/CAM systems do not generate code for avoiding constraints on the wing preform. Without that code, the stitching machine would attempt to stitch through stringers, spar caps and intercostals. Consequently, programmers must work off a geometric model of the wing cover (e.g., a loft surface), identify the constraints, sift through perhaps a million lines of CNC code, and insert appropriate instructions ensuring that the constraints are not violated.
Moreover, the CNC code must be generated by a person skilled in the art of programming CNC machines. However, each stitching machine might use codes that are unique to the stitching machine. Additionally, different programmers might take different approaches towards avoiding the constraints. The programmers can spend thousands of man-hours walking through millions of instruction codes, inserting additional instructions, and experimenting with their own routines for avoiding constraints, and testing the code.
Based on the foregoing, it can be appreciated that there exists a need for a faster, more efficient process for generating the CNC code. There also exists a need for a process that is less labor-intensive.